Heavy vehicles, such as trucks and lorries, are known which have two rear axles, namely a forward-rear axle assembly and a rear-rear axle assembly. The rear-rear axle assembly is behind the forward-rear axle assembly. Each axle is a driven axle and includes a differential unit for driving the right and left wheels associated with that axle. Engine power is transmitted through a gear box or the like along a prop shaft to a front drive flange of the forward-rear axle assembly. Housed within the forward-rear axle assembly housing is an inter-axle differential unit which transfers power to both the forward-rear differential unit (also housed within the forward-rear axle assembly housing) and the rear-rear differential unit. Power is transmitted to the forward-rear differential unit via gears and the like within the forward-rear axle housing. Power is transmitted to the rear-rear differential unit via a shaft which is mainly housed within the forward-rear axle housing but projects rearwardly out of the forward-rear axle housing. A prop shaft or the like connects the shaft to a drive flange of the rear-rear axle housing.
As will be appreciated the forward-rear axle housing includes two differential units namely the inter-axle differential unit and the forward-rear differential unit. However, the rear-rear axle housing only includes a single differential unit, namely the rear-rear axle differential unit. Such an arrangement results in a forward-rear axle housing that necessarily must be larger (to accommodate the greater number of components) than the rear-rear axle housing which can be smaller. Consequently the forward-rear axle housing is different from the rear-rear axle housing. This in turn requires two sets of manufacturing components, and two different assembly lines for assembling the two different axle housings.